As a conductor for carrying electricity, copper is being used the most at the present time. This is due to the facts that the specific resistance at room temperature is about the same degree as with silver and lowest compared with other substances and that it is relatively inexpensive. As the method for reducing the specific resistance of a conductor, there is the method of cooling the conductor. In the case of copper, if cooled to the temperature of liquid nitrogen (77K), the specific resistance becomes about one-seventh or about 2.5×10−9 Ωm.
A superconducting wire has to be cooled to under the superconducting transition temperature, but the electrical resistance is substantially zero making it an ideal conductor. A metal-based superconducting wire is highly perfected as a wire and is being used in MRI systems etc. as magnets, but has not spread widely in use due to the need for cooling to a cryogenic temperature.
On the other hand, oxide-based superconducting materials becoming superconductive at the liquid nitrogen temperature include Bi-based and Y-based materials. Bi-based materials are mainly being developed as tape wires with silver sheaths, while Y-based materials are being developed as tape wires comprised of metal tapes formed on their surfaces with buffer layers and formed on those with superconductive thin films.
These wires are increasingly promising since when high properties are obtained, they can be cooled by easy-to-handle liquid nitrogen. Further, the development and spread of electrical apparatuses using these wires can be expected. Further, as plate-shaped superconductors, NbTi and Cu multilayer plates are being developed and are being used for magnetic shields and other applications.
As oxide superconducting bulk materials having sufficient critical current densities at the liquid nitrogen temperature, materials comprised of monocrystalline REBa2Cu3O7-x (where RE is a rare earth element including Y) in which RE2BaCuO5 is finely dispersed is known. Since such materials are monocrystalline, there are technical limits in the production of large-sized materials. At the present time, only ones of a diameter of around 100 mm are known. Larger materials than this have not been obtained.
Bi-based wires, however, do not give a sufficient critical current density at 77K and in particular suffer from the problems that there is remarkable deterioration of properties in a magnetic field, that they are expensive since silver is used as a sheath material, etc. Y-based wires suffer from problems in the film forming speed in a vacuum, uniformity of properties, etc. and are still in development.